23  Unit 5: Paths of Severe Storms 5E

Why do storms follow the paths they do, and could those paths shift?

Author

Earth & Space Science

HS-ESS2-8 Time: 6–11 Days

🗺️ Why Do Storms Follow the Paths They Do? 🗺️

24 Engage: The Investigative Phenomenon

24.1 🌍 Storm Tracks: 2018–2020

Maps from 2018–2020 show that blizzards and hurricanes exhibit clear patterns in where they start and the direction they travel.

24.1.1 🤔 Key Observations:

  • 🌀 Hurricanes form over warm tropical oceans and move westward, then curve northeast
  • 🌨️ Blizzards track along the East Coast, moving from southwest to northeast
  • 🗺️ Neither storm type occurs randomly — they follow predictable pathways

Driving Question: Why do severe storms follow specific paths, and what could cause those paths to change?

24.1.2 📝 Pattern Spotting

Look at the storm track maps below. Before we explain the science, write down:

  1. What geographic patterns do you notice in storm tracks?
  2. What direction do most storms move in the mid-latitudes?
  3. Why do you think hurricanes start near the equator but blizzards start in the mid-latitudes?

25 Explore: Mapping Storm Trajectories

25.1 🔬 Investigation: Where Do Storms Go?

Let’s visualize real storm trajectory patterns and look for the underlying cause of these paths.

25.1.1 💡 Key Observation

Despite forming in very different locations and for very different reasons, both hurricanes (once they curve north) and blizzards travel in the same general direction: from southwest to northeast across the mid-latitudes.

This is NOT a coincidence. Something is steering these storms.

26 Explain: The Global Wind Engine

26.1 🌬️ Why Does the Wind Blow the Way It Does?

To understand storm paths, we need to understand the global circulation of the atmosphere — the “engine” that steers weather systems around the planet.

26.2 Uneven Solar Heating: The Root Cause

The Sun doesn’t heat Earth evenly. The equator receives far more solar energy per unit area than the poles because of the angle at which sunlight hits the surface.

26.3 Convection Cells: Nature’s Heat Redistributors

This uneven heating drives enormous convection cells in the atmosphere — rising warm air near the equator and sinking cool air at higher latitudes.

26.3.1 💡 Key Concept: Three Circulation Cells

Cell Latitude Surface Winds Key Feature
Hadley 0°–30° Trade Winds (NE in NH) Rising air at equator → heavy rain; sinking at 30° → deserts
Ferrel 30°–60° Westerlies Driven by Hadley & Polar cells; steers mid-latitude weather from W→E
Polar 60°–90° Polar Easterlies Cold, dry air sinking at poles

NYC is at ~41°N — right in the Ferrel Cell’s westerlies. This is why storms affecting our city generally move from southwest to northeast!

26.4 The Coriolis Effect: Why Winds Curve

If Earth didn’t rotate, winds would blow straight from high to low pressure. But Earth does rotate, and this deflects moving air:

  • Northern Hemisphere: winds curve to the RIGHT
  • Southern Hemisphere: winds curve to the LEFT

26.4.1 💡 Key Concept: Coriolis Effect + Pressure Gradient = Surface Winds

  • Without Coriolis: wind blows straight from H → L pressure
  • With Coriolis (NH): wind is deflected right → curves clockwise around High pressure, counter-clockwise around Low pressure
  • This is why tropical trade winds blow from the northeast (not straight south) and mid-latitude westerlies blow from the southwest (not straight north)

26.5 The Jet Stream: The Storm Highway

At the boundary between the Ferrel and Polar cells (~60°N, but it meanders), there is an extremely fast river of air flowing from west to east at high altitude: the jet stream.

26.5.1 💡 Key Concept: The Jet Stream Steers Storms

  • The polar jet stream is a band of fast-moving air (100–200+ mph) at ~30,000 ft altitude
  • It forms at the boundary between cold polar and warm tropical air
  • Mid-latitude storms ride the jet stream from west to east
  • Stronger temperature gradient → faster, straighter jet → predictable storm tracks
  • Weaker gradient (Arctic amplification) → slower, wavier jet → storms can stall, creating extended extreme events

27 Elaborate: Precipitation Patterns & Climate Connections

27.1 🌧️ Where Does It Rain (and Snow)?

Global circulation doesn’t just create wind patterns — it determines where on Earth gets rain and where stays dry.

27.2 Rising Air = Rain, Sinking Air = Dry

27.2.1 📝 Connecting Circulation to Precipitation

  1. Why does the equator receive so much rainfall? (Hint: think about the Hadley Cell)
  2. Why are the world’s great deserts located near 30°N/S? (Sahara, Arabian, Mojave…)
  3. NYC is at ~41°N. What type of precipitation mechanism dominates there?
  4. If global wind patterns shifted northward due to warming, how might precipitation patterns change for NYC?

28 Elaborate: Shifting Storm Paths

28.1 🌡️ Could Climate Change Move Storm Tracks?

If the temperature gradient between the equator and poles changes, the circulation cells — and the jet stream — could shift. What would that mean for storms?

🌍 As the planet warms, the jet stream and storm tracks are projected to shift poleward — changing which regions get storms and how intense they are! 🌍

29 Evaluate: Storm Path Analysis

29.1 ✅ Assessment: Explaining Storm Trajectories

29.1.1 🧠 Check Your Understanding

Question 1: The prevailing westerlies that steer storms across the mid-latitudes are ultimately caused by:

Question 2: Why do both hurricanes (after recurvature) and Nor’easters travel from southwest to northeast?

Question 3: How might Arctic amplification change storm paths in the future?

29.1.2 📝 Culminating Task: Storm Path Investigation

Using the concepts from this chapter, write a scientific explanation that addresses:

“Why do blizzards and hurricanes follow the paths they do, and how might those paths change in the future?”

Your explanation should reference:

  1. Uneven solar heating and how it creates atmospheric circulation cells
  2. The Coriolis effect and how it shapes surface wind patterns
  3. The jet stream and its role as a storm highway
  4. Arctic amplification and how it may alter the jet stream
  5. A prediction about how NYC’s storm exposure could change

Support your explanation with at least three pieces of evidence from data or diagrams in this chapter.

30 Summary: Key Takeaways

Concept Key Idea
Uneven heating Equator gets more solar energy → drives convection cells
Convection cells Hadley, Ferrel, Polar cells redistribute heat globally
Coriolis effect Earth’s rotation deflects winds → creates prevailing wind patterns
Westerlies Dominant winds in 30°–60° latitude → steer storms SW → NE
Jet stream Fast upper-level winds at cell boundaries → storm “highway”
Arctic amplification Reduced gradient → wavier jet stream → storms may stall or shift
Storm tracks Both hurricanes and blizzards are steered by the same global winds

Next up: We’ll investigate how hurricanes form and why they only occur during certain months. 🌀